1. Technical Field
The present invention relates to a liquid crystal device, and particularly to a transflective liquid crystal device having both the reflective display function of performing display by reflecting light, which is incident on a liquid crystal layer from a front side of liquid crystal cell, by a transflective layer, and the transmissive display function of performing display by transmitting light, which is incident on the liquid crystal layer from a back side, through the transflective layer. The present invention also relates to an electronic apparatus using the liquid crystal device.
2. Background Art
Although a reflective liquid crystal device is generally used for the display unit of a portable electronic apparatus, it has a problem in that a display cannot be recognized in the dark because of the use of external light incident on a liquid crystal layer from the front side of a liquid crystal cell. Therefore, a transflective liquid crystal device has been proposed in which a display can be recognized by using external light, like a reflective liquid crystal device, in the light, and using light emitted from an illumination device arranged on the back side of a liquid crystal cell in the dark.
The transflective liquid crystal device comprises a polarizer, a transflector, and the illumination device, which are arranged in this order on the back side of the liquid crystal cell, as disclosed in Japanese Unexamined Utility Model Publication No. 57-49271. In this liquid crystal device, in bright reflective display is performed by reflecting the external light incident on the liquid crystal layer from the front side of the liquid crystal cell by a transflector, and in dark surroundings, transmissive display is performed by transmitting the light emitted from the illumination device through the transflector.
An example of other transflective liquid crystal devices is the transflective liquid crystal device disclosed in Japanese Unexamined Patent Publication No. 8-292413, which is aimed at improving the brightness of reflective display. This transflective liquid crystal device comprises a transflector, a retardation plate, a polarizer, and a back light, which are arranged on the back side of the liquid crystal cell in this order, wherein in light surroundings, external light incident on the liquid crystal layer from the front side of the liquid crystal cell is reflected by the transflector to perform reflective display, and in dark surroundings, the light emitted from the back light is transmitted through the transflector to perform transmissive display. Such a structure produces brighter reflective display than the aforementioned liquid crystal device because no polarizer is present between the liquid crystal cell and the transflector.
However, the transflective liquid crystal device disclosed in the above publication comprises a transparent substrate interposed between the liquid crystal layer and the transflector, and thus causes a double image due to parallax when reflective display is performed. Particularly, a color liquid crystal device comprising a combination of the transflective liquid crystal device disclosed in the above publication and color filters has the high possibility that the color filter through which light incident on the liquid crystal layer from the front side of the liquid crystal cell passes, and the color filter through which the incident light passes after being reflected by the transflector are different, thereby causing the problem of paling a display color.
In order to solve this problem, Japanese Unexamined Patent Publication Nos. 7-318929 and 7-333598 disclose the invention of a transflective liquid crystal device including a transflector arranged in a liquid crystal cell in order to remove parallax.
In the transflective liquid crystal devices disclosed in Japanese Unexamined Patent Publication Nos. 7-318929 and 7-333598, i.e., transflective liquid crystal devices performing reflective display without using the polarizer provided on the back side of the liquid crystal cell, when the light, which is incident from the front side of the liquid crystal cell and passes through the liquid crystal layer is reflected by the transflector, the light is preferably changed to circularly polarized light or elliptically polarized light with high ellipticity in a dark display state, and changed to linearly polarized light or elliptically polarized light with low ellipticity in a bright display state. This is because when circularly polarized light or elliptically polarized light with high ellipticity, which is reflected by the transflector, again passes through the liquid crystal layer, the light is changed to linearly polarized light perpendicular to the transmission axis of the polarizer provided on the front side of the liquid crystal cell or elliptically polarized light with low ellipticity, and absorbed by the polarizer, thereby realizing good contrast properties.
On the other hand, the light transmitted through the transflector from the back side of the liquid crystal cell is constantly in the same polarization state without dependency of the display states.
In the transflective liquid crystal devices disclosed in Japanese Unexamined Patent Publication Nos. 7-318929 and 7-333598, no optical element for changing polarization of the light incident on the liquid crystal layer is provided between the polarizer provided on the back side of the liquid crystal cell and the transflector, and thus linearly polarized light transmitted through the polarizer provided on the back side of the liquid crystal cell is constantly incident on the liquid crystal layer. Therefore, by preferable setting for reflective display in which the light reflected by the transflector is changed to circularly polarized light or elliptically polarized light with high ellipticity in the dark display state, the contrast properties of transmissive display deteriorate.
This is because linearly polarized light incident on the liquid crystal layer from the back side of the liquid crystal cell, in the dark display state, is changed to circularly polarized light or elliptically polarized light with high ellipticity when passing through the liquid crystal layer, and thus part of the light is transmitted through the polarizer provided on the front side of the liquid crystal cell.
The present invention has been achieved for solving the above problems, and a first object of the invention is to obtain a transflective liquid crystal device exhibiting good contrast properties in transmissive display, and a second object of the invention is to obtain a transflective liquid crystal device causing no double image due to parallax.
In order to achieve the objects, the liquid crystal device according to the present invention has a reflective display function of performing display by reflecting light, which is incident on a liquid crystal layer from one side of liquid crystal cell, by a transflective layer, and the transmissive display function of performing display by transmitting light, which is incident on the liquid crystal layer from the other side opposite to the one side, through the transflective layer. According to the liquid crystal device, a first display state as a bright display state and a second display state as a dark display state can be selected by changing the voltage applied to the liquid crystal layer; and in the second display state, the light incident on the liquid crystal layer from the one side of the liquid crystal cell passes through the liquid crystal layer and is reflected by the transflective layer to be changed to circularly polarized light or elliptically polarized light with the predetermined rotational direction. The liquid crystal device has a first polarizer provided on the one side, and an optical element provided on the other side, for polarizing the light incident on the transflective layer from the other side into a light with a predetermined rotational direction.
In the first liquid crystal device, the transflective layer is a layer for reflecting and transmitting incident light with certain reflectance and transmittance, which is formed by, for example, a commercially available half mirror, a metal film having an aperture, a metal film formed as thin as part of light can be transmitted.
In an embodiment of the first liquid crystal device, reflective display is performed by using light incident on the liquid crystal layer from the one side, i.e., the front side of the liquid crystal cell. In this case, linearly polarized light produced by polarization by the first polarizer is incident on the liquid crystal layer from the front side of the liquid crystal cell, passed through the liquid crystal layer, reflected by the transflective layer, and then again passed through the liquid crystal layer. The light transmitted through the first polarizer is emitted as image light from the front side of the liquid crystal device. In this liquid crystal device, when the quantity of light emitted from the front side of the liquid crystal cell is small, for example, in the dark, transmissive display is performed by using light incident from the other side, i.e., the back of the liquid crystal cell. In this case, the light incident from the other side is transmitted through the transflective layer, and then passes through the liquid crystal layer. The light transmitted through the first polarizer is emitted as image light from the front side of the liquid crystal device.
In the first liquid crystal device, in the dark display state, the light incident from the front side of the liquid crystal cell passes through the liquid crystal layer, and is then reflected by the transflective film to be changed to circularly polarized light or elliptically polarized light with the predetermined rotational direction. The light again passes through the liquid crystal layer to be changed to linearly polarized light in the direction perpendicular to the transmission axis of the first polarizer, or elliptically polarized light with the long axis direction different from the transmission axis of the first polarizer, and is thus absorbed by the first polarizer.
On the other hand, the light incident from the back of the liquid crystal cell is changed by the optical element to light with the predetermined rotational direction, i.e., the same rotational direction as the light incident from the front side of the liquid crystal cell and then is reflected by the transflective layer. Then, the light passes through the transflective layer. In passing through the liquid crystal layer, the light incident from the back of the liquid crystal cell is changed to linearly polarized light perpendicular to the transmission axis of the first polarizer or elliptically polarized light having the long axis direction different from the transmission axis of the first polarizer, and is thus absorbed by the first polarizer.
Namely, in the dark display state, the polarization state of the light emitted from the liquid crystal layer toward the first polarizer in reflective display is the same as or close to that in transmissive display, thereby preventing the contrast of the transmissive display from being decreased due to a difference in polarization state between the both.
In another embodiment of the first liquid crystal device of the present invention, in the aforementioned second display state, the ellipticity in reflection of the light, which is incident on the liquid crystal layer from the one side of the liquid cell and reflected by the transflective layer, is substantially the same as the ellipticity in transmission of the light, which is incident on the liquid crystal layer from the other side of liquid crystal cell and transmitted through the transflective layer.
In this embodiment, in the dark display state, the polarization state of the light emitted from the front side of the liquid crystal cell in reflective display is the same as that in transmissive display, thereby preventing a decrease in the contrast of the transmissive display.
In a further embodiment of the first liquid crystal device of the present invention, a bright display state as a first display state, a dark display state as a second display state, and a medium brightness state is between the first and second states as a third display state can be selected by changing the voltage applied to the liquid crystal layer. In this embodiment, the third display state not only shows the specified brightness but includes a plurality of display states which can be selected according to the voltage applied to the liquid crystal layer.
In this embodiment, the brightness display state, the dark display state, and the medium brightness display state can be selected, thereby permitting so-called halftone display.
In a still further embodiment of the first liquid crystal device, in the second display state, the light incident on the liquid crystal layer from the one side of liquid crystal cell is reflected by the transflective film to be changed to circularly polarized light with the predetermined rotational direction, and in the first display state, the light incident on the liquid crystal layer from the one side of liquid crystal cell is reflected by the transflective film to be changed to linearly polarized light.
In this embodiment, in the second display state, light incident on liquid crystal layer from the the front side of the liquid crystal cell is reflected by the transflective film to be changed to circularly polarized light. The circularly polarized light reflected by the transflective layer passes through the liquid layer again to be changed to linearly polarized light perpendicular to the transmission axis of the first polarizer, which is about 100% absorbed by the first polarizer. On the other hand, in the first display state, the light incident on the liquid crystal layer from the front side of the liquid crystal cell is reflected by the transflective film without a change in the polarization state, again passes through the liquid crystal layer, and then transmitted through the first polarizer. Therefore, reflective display having highest utilization efficiency of light and optimum contrast properties is realized. In this case, when the light incident on the liquid crystal layer from the back side of the liquid crystal cell is transmitted through the transflective layer and circulary polarized, the transmissive display exhibits highest contrast.
In a further embodiment of the first liquid crystal device, the optical element comprises a second polarizer or a reflective polarizer provided on the other side, and a retardation plate provided between the second polarizer or reflective polarizer and the liquid crystal cell. As the second polarizer used in this embodiment, a polarizer having the function to transmit a linearly polarized light component in a certain direction and absorb a linearly polarized light component perpendicular to this direction can be used. As the reflective polarizer, a reflective polarizer having the function to transmit a linearly polarized light component in a certain direction and reflect a linearly polarized light component perpendicular to this direction, can be used. Such a reflective polarizer is disclosed in detail in International Publication No. WO095/01788.
In a further embodiment of the first liquid crystal device, the transmission axis of the polarizer or reflective polarizer, and the axis and retardation value of the retardation plate are set so that when the light incident on the liquid crystal layer from the other side is transmitted through the transflective layer, the ellipticity is 0.85 or more.
In this embodiment, the light incident on the liquid crystal layer from the back of the liquid crystal cell is changed to circularly polarized light or elliptically polarized light with high ellipticity (i.e., close to circularly polarized light), thereby realizing the first liquid crystal device exhibiting higher contrast in the transmissive display.
In a further embodiment of the first liquid crystal device, the retardation plate comprises a 1/4 wavelength plate.
In this embodiment, the retardation plate comprises the 1/4 wavelength plate, and thus the light changed to linearly polarized light by the second polarizer can be incident as completely circularly polarized light on the transflective layer. Japanese Unexamined Patent Publication No. 5-100114 discloses that circularly polarized light can also be obtained by a method using a wide-band circular polarizer comprising a lamination of a 1/2 wavelength plate and a 1/4 wavelength plate, a method using a 3/4 wavelength plate or a 5/4 wavelength plate, or the like. However, according to the latter methods, the wavelength region for good circularly polarized light is narrow, and thus a method using a single 1/4 wavelength plate is preferably used.
In a further embodiment of the first liquid crystal device, a liquid crystal polymer exhibiting a cholesteric phase is used as the optical element. Such a liquid crystal polymer has the function to selectively reflect and transmit circularly polarized light according to its rotational direction. The liquid crystal polymer is disclosed in detail in Japanese Unexamined Patent Publication No. 8-27189.
In a further embodiment of the first liquid crystal device, an illumination device is further provided on the side of the optical element different from the liquid crystal layer side.
In this embodiment, light emitted from the illumination device can be incident on the liquid crystal cell from the back side of the liquid crystal cell, thereby permitting transmissive display by using the light from the illumination device in the use of the liquid crystal device in the dark.
A second liquid crystal device of the present invention comprises a liquid crystal cell having a liquid crystal layer held between a first substrate and a second substrate arranged opposite to the first substrate; a transflective layer arranged on the liquid crystal layer side of the second substrate for reflecting and transmitting incident light with predetermined reflectance and transmittance; an illumination device arranged on the side of the second substrate different from the liquid crystal layer side; a polarizer or reflective polarizer arranged between the liquid crystal cell and the illumination device; and a retardation plate arranged between the polarizer and the liquid crystal cell for changing the linearly polarized light, resulting from the light emitted from the illumination device and then transmitted through the polarizer, to circularly polarized light or elliptically polarized light; wherein the rotational direction of the circularly polarized light or the elliptically polarized light resulting from the light emitted from the illumination device and then transmitted through the retardation plate is the same as that of the circularly polarized light or the elliptically polarized light resulting from the light incident on the liquid crystal layer from the first substrate side and then reflected by the transflector in the dark display state.
The transflective layer used in the second liquid crystal device is a layer reflecting and transmitting light with predetermined reflectance and transmittance, and for example, a metal film having a very narrow slit, a metal thin film, or the like is suitable as the transflector.
In the second liquid crystal device of the present invention, the light emitted from the illumination device and then transmitted through the second polarizer to be changed to linearly polarized light is changed to circularly polarized light or elliptically polarized light by the retardation plate. The rotational direction of the circularly polarized light or elliptically polarized light is the same as the rotational direction of circularly polarized light resulting from the light insident on the liquid crystal layer from the first polarizer side, then passes through the liquid crystal layer in the dark display state and reflected by the transflective layer. Therefore, high contrast properties are realized in transmissive display. Furthermore, since no substrate is interposed between the transflective layer and the liquid crystal layer, there is no problem of a double image due to parallax in reflective display.
In an embodiment of the second liquid crystal device, the transmission axis of the second polarizer or reflective polarizer and the axis and retardation value of the retardation plate are set so that polarized light resulting from the light emitted from the illumination device and then transmitted through the retardation plate has an ellipticity of 0.85 or more.
In another embodiment of the second liquid crystal device, the retardation plate comprises at least one 1/4 wavelength plate. Japanese Unexamined Patent Publication No. 5-100114 discloses that circularly polarized light can also be obtained by a method using a wide-band circular polarizer comprising a lamination of a 1/2 wavelength plate and a 1/4 wavelength plate, a method using a 3/4 wavelength plate or a 5/4 wavelength plate, or the like. However, according to the latter methods the wavelength region for good circularly polarized light is narrow, and thus a method using a single 1/4 wavelength plate is preferably used.
A third liquid crystal device of the present invention comprises a liquid crystal cell having a liquid crystal layer held between a first substrate and a second substrate arranged opposite to the first substrate; a transflector arranged on the liquid crystal layer side of the second substrate for reflecting and transmitting incident light with predetermined reflectance and transmittance; an illumination device arranged on the side of the second substrate different from the liquid crystal side; and a selective reflective layer arranged between the liquid crystal cell and the illumination device for selectively reflecting and transmitting circularly polarized light or elliptically polarized light according to its rotational direction; wherein the rotational direction of the circularly polarized light resulting from the light emitted from the illumination device and then transmitted through the selective reflective layer is the same as that of the circularly polarized light resulting from the light incident on the liquid crystal layer from the first substrate side and then reflected by the transflector, in the dark display.
In the third liquid crystal device of the present invention, of light emitted from the illumination device, circularly polarized light or elliptically polarized light with the predetermined rotational direction is transmitted through the selective reflective layer. The rotational direction of the circularly polarized light or the elliptically polarized light is the same as that of the circularly polarized light or the elliptically polarized light resulting from the light emitted from the first substrate side, then passing through the liquid crystal layer in the dark display state, and reflected by the trasflector. Therefore, high contrast properties are realized in transmissive display. In addition, since no substrate is interposed between the transflective layer and the liquid crystal layer, there is no problem of a double image due to parallax in reflective display. Further, part of the light reflected by the selective reflective layer is also transmitted through the selective reflective layer due to scattering by the surface of the illumination device, thereby increasing the utilization efficiency of light emitted from the illumination device.
In an embodiment of the third liquid crystal device, the selective reflective layer is, for example, a reflective layer which employs the selective reflection of a comprising a cholesteric liquid crystal.
In another embodiment of the third liquid crystal device, the selective reflective layer is a film-shaped circularly polarized light reflector which employs the selective reflection of a cholesteric liquid crystal, and has the function to transmit right-handed circularly polarized light and reflect left-handed circularly polarized light, or transmit left-handed circularly polarized light and reflect right-handed circularly polarized light. Details of such a selective reflective layer are disclosed in Japanese Unexamined Patent Publication No. 8-27189.
An electronic apparatus of the present invention is an electronic apparatus comprising a liquid crystal device as a display unit, wherein the first, second or third liquid crystal device is provided as the liquid crystal device.
In an electronic apparatus comprising the first liquid crystal device, good contrast properties are realized in transmissive display.
In an electronic apparatus comprising the second or third liquid crystal device, good contrast properties are realized, and there is no double image due to parallax, in transmissive display.
In the first, second and third liquid crystal devices of the present invention, the rotational direction of circularly polarized light or elliptically polarized light means the rotational direction of an electric field vector of light. Namely, xe2x80x9cleftxe2x80x9d and xe2x80x9crightxe2x80x9d in so-called xe2x80x9cleft-handed polarized lightxe2x80x9d and xe2x80x9cright-handed polarized lightxe2x80x9d indicate the rotational direction.
In the first, second and third liquid crystal devices of the present invention, the liquid crystal layer in the dark display state represents the liquid crystal layer to which a voltage necessary for obtaining sufficiently dark display is applied. Namely, in normally black display mode, the liquid crystal layer in the dark display state means a liquid crystal layer with no voltage applied or the non-selective voltage applied, while in normally white display mode, it means a liquid crystal layer with the selective voltage applied.
In the first, second and third liquid crystal devices according to the present invention, the objects of the present invention can be achieved as long as the light changed to circularly polarized light or elliptically polarized light is light within the predetermined wavelength range in the visible wavelength region. However, it is preferable that the circularly polarized light or elliptically polarized light with high ellipticity is obtained at the vicinity of the maximum strength wavelength in case that the light emitted from the illumination device is colored light, while the same is obtained at at the green wavelength where the human visibility is highest in case that the light emitted from the illumination device is white light. Of course, it is ideal to obtain uniform circularly polarized light or elliptically polarized light with high ellipticity at all wavelengths over the entire visible wavelength region.
In the first, second and third liquid crystal devices of the present invention, in order to secure a contrast, it is optimum to obtain circularly polarized light, but circularly polarized light is sometimes intentionally shifted to improve the brightness of transmissive display. With an ellipticity of less than 0.85, the contrast of transmissive display is lower than that of reflective display.
The display operation of the liquid crystal device of the present invention will be described in further detail below.
First, description is made on the reason why the light incident from the second polarizer side is changed to circularly polarized light or elliptically polarized light in transmission through the transflector, and the reason why the contrast of transmissive display is increased when the rotational direction of that circularly or elliptically polarized light is the same as the rotational direction of the circularly polarized light or elliptically polarized light resulting from the light incident on the liquid crystal layer from the first polalizer side, then passing through the liquid crystal layer in the dark display state, and reflected by the transflector. Although the description below is made on the assumption that the light incident from the second polarizer side is changed to circularly polarized light in transmission through the transflector, the basic principle is the same when the incident light is changed to elliptically polarized light.
On the basis of FIG. 12, description is made on the fact that in reflective liquid crystal display mode of the single-polarizer type, which is used in the first, second and third liquid crystal devices of the present invention, the light incident from the first polarizer side is changed to circularly polarized light when passing through the liquid crystal layer in the dark display state and then reflected by the transflective surface.
FIG. 12(a) shows a single-polarizer type reflective liquid crystal device. Reference numeral 1201 denotes a polarizer; reference numeral 1202, a reflector; and reference numeral 1211, a liquid crystal layer in the dark display state. Members such as a substrate, an alignment film, a transparent electrode, etc. are not required for describing the operation, and are thus omitted. A retardation plate may be provided between the polarizer and the liquid crystal layer. However, if the retardation plate is considered as a first layer of a liquid crystal layer, the description below is applicable, and thus the retardation plate is also omitted.
In the dark display state, the linearly polarized light incident on the liquid crystal layer from the polarizer 1201 passes forward and backward through the liquid crystal layer 1211 to be changed to linearly polarized light perpendicular to the incident polarized light, and is then absorbed by the polarizer 1201.
FIG. 12(b) shows a structure in which a polarizer 1204 and a liquid crystal layer 1212 are arranged so as to have mirror symmetry to the polarizer 1201 and the liquid crystal layer 1211 with the phantom central plane 1203 held therebetween. The single-polarizer type reflective liquid crystal device shown in FIG. 12(a) is equivalent to the two-polarizer type transmissive liquid crystal device shown in FIG. 12(b). The linearly polarized light incident on the liquid crystal layer from the polarizer 1201 is changed to linearly polarized light perpendicular to the incident light by the liquid crystal layers 1211 and 1212, and then absorbed by the polarizer 1204.
Here a structure is assumed. FIG. 12(c) shows a structure in which the liquid crystal layer 1212 and the polarizer 1204 shown in FIG. 12(b) are rotated 90xc2x0 to be changed to a liquid crystal layer 1213 and a polarizer 1205, respectively. In this structure, the liquid crystal layers symmetrical across the central plane 1203 held therebetween are perpendicular to each other as long as they are seen from at least the normal of the central plane. Namely, the fast axis and the slow axis are overlapped to compensate for retardation. Therefore, the linearly polarized light incident on the liquid crystal layer from the polarizer 1201 is changed to various types of light by the liquid crystal layers 1211 and 1212, then returned to the initial linearly polarized light, and absorbed by the polarizer 1205 to produce dark display.
If the structure shown in FIG. 12(b) is equivalent to the structure shown in FIG. 12(c), the structure shown in FIG. 12(c) is certainly put into the dark display. A requirement for making both structures equivalent is that the polarization state in the central plane 1203 is not changed even by 90xc2x0 rotation. The polarization states meeting such requirement are only two, i.e., right-handed circularly polarized light and left-handed circularly polarized light. Therefore, the light passing through the liquid crystal layer in the dark display state is changed to circularly polarized light and reaches the reflection surface.
If the above description is clarified, residual description can easily be made. In FIG. 13, reflective display is performed as described below. Incident light 1311 from the outside is changed to linearly polarized light 1321 when passing through a polarizer 1301. The linearly polarized light is changed to, for example, right-handed circularly polarized light 1322 when passing through a liquid crystal layer in the dark display state, and then reaches a transflector 1302. The right-handed circularly polarized light 1322 is reflected by the transflector 1302 to be changed to left-handed circularly polarized light 1322 with a change in the travel direction of light. The light 1322 again passes through the liquid crystal layer in the dark display state to be changed to linearly polarized light 1323, and then absorbed by the polarizer 1301.
In order to obtain high contrast in the transmissive display, the dark display must be sufficiently dark. Namely, when passing through the transflector, the incident light 1312 from the back is preferably changed to the same left-handed circularly polarized light 1322 as the case of the reflective display.
Conversely, if the incident light 1312 is changed to right-handed circularly polarized light, bright display is produced as negative display in which light and darkness are reverse to the reflective display.